It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The strong believer is better and more beloved to Allah than the weak believer, though there is good in both. Be eager for what benefits you, seek help from Allah and do not give up. If anything befalls you, do not say: If only I had done such and such; rather say: 'Qadar Allah wa ma sha'a fa'al' (Allah decreed, and what He willed He did), for the word 'if' opens the work of Satan."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20173